Subscriber line interface circuits are typically found in the central office exchange of a telecommunications network. A subscriber line interface circuit (SLIC) provides a communications interface between the digital switching network of a central office and an analog subscriber line. The analog subscriber line connects to a subscriber station or telephone instrument at a location remote from the central office exchange.
The analog subscriber line and subscriber equipment form a subscriber loop. The interface requirements of a SLIC result in the need to provide relatively high voltages and currents for control signaling with respect to the subscriber equipment on the subscriber loop. Voiceband communications are low voltage analog signals on the subscriber loop. Thus the SLIC must detect and transform low voltage analog signals into digital data for transmitting communications received from the subscriber equipment to the digital network. For bi-directional communication, the SLIC must also transform digital data received from the digital network into low voltage analog signals for transmission on the subscriber loop to the subscriber equipment.
A subscriber line interface circuit requires different power supply levels depending upon operational state. One supply level is required when the subscriber equipment is “on hook” and another supply level is required when the subscriber equipment is “off hook”. Yet another supply level is required for “ringing”.
The SLIC must be provided with a negative voltage supply sufficient to accommodate the most negative loop voltage while maintaining the SLIC internal circuitry in their normal region of operation. In order to ensure sufficient supply levels, a power supply providing a constant or fixed supply level sufficient to meet or exceed the requirements of all of these states may be provided. The use of a single fixed negative power supply tends to result in unnecessary power dissipation.
Another solution is to provide multiple fixed power supplies, each associated with a particular state of the subscriber equipment. The SLIC automatically selects between the multiple fixed power supplies depending upon the state of the subscriber equipment. Although some unnecessary power dissipation may be alleviated, the use of multiple fixed power supplies is disadvantageous. Aside from the inconvenience of multiple supplies, such granularity provides only a coarse reduction of the power dissipation.